By Slowhand
United Kingdom 
in my greenhouse there are hundreds of small black flies moving around on the compost they are not aphids I have sprayed them with bug killier and puffed them with sybol dust but they are not effected .Have you any ideas what they are and do they harm seedlings.
1 Oct, 2011
Answers
Could be an indication that the compost it too wet
1 Oct, 2011
Probably fungus gnats. The larvae eat fungus in the compost, and, as MG says, they are often a symptom of overwatering, and/or too heavy compost. I have found that organic mosquito granules containing Bacillus thuringiensis v. israelensis often remove them.
1 Oct, 2011
But why, Tug? They are an indicator of the gardeners (including myself) poor practices and are there to help us by eating the fungus growing on the compost. Next year we just might do better and make better compost; or not:-(
1 Oct, 2011
Trruue...but I do get tired of inhaling the little b-------s! ; )
2 Oct, 2011
Previous question
I don't know just what they are but they are very common and do not seem to do any harm. We get lots but they have not caused us any problem.
1 Oct, 2011